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Author Topic: Best Amplifier for AM  (Read 4342 times)
AC0FA
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« on: July 26, 2007, 08:54:59 AM »

Hi, I have kind of a combination Boatanchor/amplifier question. I have a vintage AM rig that the absolute peak am output power will be 25W PEP.

I would like to incerase power to 500w pep.
what kind of an amplifier do I need?

Is there a diference between an SSB amp and an AM amp?

Or are they all the same usual SSB amps just rubbibg AM through them?

For Example; Heathkit SB 220 does 1000w  SSB and 400w am.

John          
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KE3WD
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« Reply #1 on: July 26, 2007, 09:03:26 AM »

The amps you cite will certainly work as linear amps with AM modulation.  

The boatanchor AM situation is often there because of the high level modulation with modulation transformer in series with PA plate supply implementation, though.  This yields that big wonderful sound that many prefer.  

Use of the linear amp along with that older rig may not yield that kind of modulation tone over the air, it may be a little bit better sounding than a modern low-level modulated solid state rig on AM, but will still have that "linear amp" sound to its modulation.  

Still, you can use it.  

Don't be surprised to find out that you get better audio reports on AM barefoot with that old rig, though.  Even though it is "only" 25W or so, the modulation will be high level modulation.  

I've noticed that the sound of high level modulation is something that the older operators appear to appreciate more than younger generations who often say that they cannot tell a difference between the two by listening.  I chalk that up to ears that have become more accustomed to things like low level AM broadcast radio, digital sound reproduction, etc. more than anything else.  I know for a fact that I could pass a double blindfold test on the subject.  

"Best" amplifier for AM would be any good and well designed HF linear amplifier BTW.  


.
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KE3WD
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« Reply #2 on: July 26, 2007, 09:07:11 AM »

BTW when setting up the linear amp in AM mode, don't try to get the watt needle to indicate full power on the carrier.  This is likely the biggest mistake of the noob on AM with an amp.  

Dead carrier should be a lot lower than where the needle swings to when you talk.  Around 40% of the full swing.  

This will give you the best audio within the passband.  "Forward Modulation" as the old timers used to call it, where their S meter swings higher on your modulation peaks.  

Of course, there is a point where this can be overdone and you want to avoid that, too.  

Peak Reading Wattmeter is handy for this, but not essential.  


.
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G3RZP
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« Reply #3 on: August 02, 2007, 01:49:17 AM »

The carrier should be no more than one quarter of the amplifier PEP. So a 500 watt amplifier should not be pushed to more than 125 watts of carrier. In practice, you might get a bit more without causing too much splatter, but it's better to hold the power back.
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W5RKL
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« Reply #4 on: August 03, 2007, 08:01:19 AM »

I agree with the previous post. In a plate modulated AM signal, half the power is in the sidebands (1/4 in each sideband). However, in sideband, "all" of the power is in the sidebands.

In some transmitters that have AM mode, AM is in the lower sideband only. The Drake T4X and TR3/4 series transmitters and transceivers along with the Heathkit Marauder HX-10, for example. These transmitters use controlled carrier screen modulation where the carrier is quite low and increases with audio, quite similiar to sideband but with a small amount of carrier. In fact, controlled carrier screen modulation can be mistakened as sideband by inexperienced operators who look for a lot of carrier in AM.

However, any way you slice it, the important thing to remember whether you are operating plate modulation or controlled carrier screen modulation is that AM mode is 100% duty cycle. Many, not all, sideband amplifiers are not designed for longwinded 100% duty cycle. With 100% duty cycle at high power levels, sideband amplifier tank circuits can fail due to excessive heat generated in the tank circuit coil. Sideband amplifiers have smaller diameter wire for the final tank coil than an AM amplifier has and the heat generated by a high power level and long winded AM signal can cause the coil to overheat and the plastic  support to melt.

To get an idea of what I am talking about, compare the wire size of a sideband transmitter's final tank coil to that of say a plate modulated full carrier Heathkit Apache TX-1. Both transmitters are rated at 100 watts. However, there is a big difference in wire size. The same applies with sideband verses AM amplifiers; SB-220 and the Heathkit Warrior amplifier.

Keep the power level down and limit transmission time when using sideband amplifiers. Yes, there are amplifier manufacturers such as the Ameritron that will withstand an AM signal. However, read the CAUTION included in the manuals, they're there for a reason.

Mike W5RKL
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KD7HVL
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« Reply #5 on: August 08, 2007, 03:10:48 PM »

This is my opinion and will most likely get rained upon by others.  The combination you are attempting to put together will be a low performance band aid at best.  the audio will be very close to what was called carrier controled. The best way I can describe to you what your going to get is take the Heathkit apache transmitter, it has a very nice audio sound due to high level plate modulation.  Now when you marry it with the 1KW amp I think it was the HA10 (warrior), also made by heathkit (used 4 X 811s'), you realized a higher s meter reading at the receive station, but the audio no longer had the punch nor fidelity. Now if you modified the HA 10 with a outboard 500 watt external modulator (which you would have had to build youself), you again restored the nice plate modulated sound and retained the fidelity, along with gaining the punch. My sugestion would be to enjoy what you have, save your bucks and get a high powered plate modulated AM transmitter. My 2 cents, gud luck in your endovers which ever way you go.
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W5WSS
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« Reply #6 on: July 06, 2008, 11:18:07 PM »

The best am transmitter for amateur use was the johnson invader with a pair of eimac 4-400a yep full duty cycle
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KX5JT
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« Reply #7 on: February 04, 2010, 02:36:08 AM »

A good linear amplifier will not change the way the driving transmitter's audio sounds.  It will faithfully reproduce it.  I have heard some Johnson Rangers with modified and beautiful plate modulated AM that sounded just as beautiful after driving a clean linear amplifier.  In fact, because the signal was stronger and the carrier quieted the frequency even more after gaining 10 db, it sounded way BETTER.  Yes, plate modulation has an awesome sound.  That sound is faithfully reproduced in a good clean linear amplifier.

Sometimes people love to believe old myths but I rather believe what I hear on the air with my own ears.
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W8JI
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« Reply #8 on: February 04, 2010, 05:02:40 AM »

There sure are a lot of myths here in the replies, with some accurate information buried within.

http://www.w8ji.com/amplitude_modulation.htm

The traditional "big iron" Ham transmitter has relatively poor distortion and distortion-bandwidth characteristics. This is because they usually plate modulate a tetrode, and tetrodes are notoriously non-linear when modulated. What nearly everyone goes by is the sound they hear, and most people can't even detect 10% distortion.

Plate modulated BC transmitters are different. While a few do use plate modulated tetrodes, they have special RF envelope detection and feedback or other special circuits to make them more linear. Most of the better high level modulated AM transmitters are plate modulated low-mu triodes, which are much more linear when plate modulated. 

Low level modulation can actually have much less distortion and more fidelity, and more faithfully reproduce the audio input. Some of the cleanest AM BC transmitters ever built were low level modulated. Unfortunately they have low efficiency and high operating cost.

There is absolutely NO difference in the sound of any AM transmitter when run through a properly tuned and operated linear amplifier. This is because a properly tuned and operated linear, be it a Heath SB220 or anything else, has much less distortion than the typical boat anchor rig. The idea your system has to be plate modulated at the antenna for best sound is an absolute myth.

The problem with a linear amp is efficiency. In order to reproduce the input faithfully, the amplifier has to be loaded to handle the PEAK power. This is normally four times the carrier power (or more in some cases). This is because the linear has to be "efficiency modulated". When at peaks the amplifier will be at peak plate efficiency, let's say about 70% at the tube and 2% tank and other losses for 68% total efficiency.

In this example at carrier plate efficiency will be about 35%, and with tank losses of 2% we have overall 33% efficiency. That means on carrier 2/3 of the plate input power will be heat, or twice as much heat as carrier power. If you ran 500 watts PEP you would have 125 watts carrier or 375 watts heat.

A safe estimate is 25% carrier efficiency. This means your amp would be making three times the heat as carrier power. An SB220 can safely handle about 500 watts of steady dissipation (inadequate airflow to fully use the tubes) so it is safe at 125 watts carrier when properly tuned.

Very few amps will handle legal limit AM. That would be 375 watts carrier, and a safe estimate would be three times that for dissipation. That would be 1125 watts of heat, which would take a lot of air and a big tube. An 8877 at full rated airflow would work.

The real problem with a linear is NOT the sound. That is an absolute myth. The real problem is heat.

A rig certainly does NOT need to be plate modulated to sound perfect, and as a matter of fact most amateur plate modulated transmitters have terrible distortion as a percentage of modulation. It's just that most people can't actually hear the distortion, they listen to and enjoy the frequency response and might actually "like" a little distortion, and they confuse distortion with good sound.

It's certainly possible to have bad low level modulation, but plate modulating a tetrode guarantees you have to do a lot of special tuning and tricks to not have significant distortion.

Tom
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WB2WIK
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« Reply #9 on: February 04, 2010, 10:43:06 AM »

>RE: Best Amplifier for AM       Reply
by W5WSS on July 6, 2008    Mail this to a friend!
The best am transmitter for amateur use was the johnson invader with a pair of eimac 4-400a yep full duty cycle<

Huh

The Johnson Invader 2000 is an SSB transmitter.  It runs AM by using double sideband injected carrier, and in no way is plate modulated.  The pair of 4-400s operate at 800W *input* power on AM, resulting in about 400W output power and get mighty hot doing that!

The Collins KW-1 was a real AM transmitter!  So was the Globe King.

But I agree with Tom (W8JI), it is absolutely impossible to tell the difference, with any amount of instrumentation, between a properly modulated low power exciter followed by a very linear amplifier and a high-level (plate/screen) modulated high power transmitter.  They both produce the same envelope, and could produce exactly the same modulation characteristics; depends on the exciter.
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W8JI
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« Reply #10 on: February 04, 2010, 01:23:05 PM »

The maximum carrier efficiency of a rig like the Invader 2000 is about 30% or less. That means tube dissipation is at least 2-3 times the carrier power.

If you ran 400 watts carrier, you would have 800-1200 watts heat in the anodes of the 4-400's.

It would however have better audio than most plate modulated rigs.
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WB4IUY
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« Reply #11 on: February 23, 2010, 07:08:43 PM »

Another good amp for AM is the old Gonset GSB-201 with 4ea. 574-B's. They're cheap as dirt, built like a tank, and work great on AM.

Dave WB4IUY
www.WB4IUY.net
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K8OT
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« Reply #12 on: February 24, 2010, 07:53:10 PM »

Johnson Thunderbolt hands down it's made for it.
ED K8OT
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W8JI
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« Reply #13 on: February 25, 2010, 04:48:03 AM »

Another good amp for AM is the old Gonset GSB-201 with 4ea. 574-B's. They're cheap as dirt, built like a tank, and work great on AM.

Dave WB4IUY
www.WB4IUY.net

4 572B's safely dissipate about 600 watts in short duty cycle, or 400 watts for long duty cycle with enough airflow.

Tuned for linear undistorted AM, with a carrier power of 130 watts, tube dissipation is about 390 watts.

The GSB201 will probably be OK at 130 watts carrier.




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W8JI
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« Reply #14 on: February 25, 2010, 04:51:41 AM »

Johnson Thunderbolt hands down it's made for it.
ED K8OT

The Johnson Thunderbolt uses 4-400 tubes.

Assuming enough airflow, the maximum carrier output power for 400 watts dissipation per tube is about 260 watts.
That would be right at the tube limits. 200 watts carrier would be safer.


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